The invention relates to the refrigeration engineering, and more particularly it deals with methods and cooling agent for freezing and storing products.
The invention may be used in the food industry, in the household and medicine for cooling and freezing, and also for a short-and long-term storage of any products, both food and biological, as well as in other fields of technology wherever it is necessary to obtain and maintain cold at a level of minus 24.degree. C. and below at minimum energy requirements.
A method for freezing and storing products in home compression refrigerators is widely known. It consists in loading products into one or several refrigerator compartments. The following temperature conditions are provided in the compartments; a temperature in the compartment for freezing and long-term storage in the regime of freezing shall not be above -24.degree. C. and maximum -18.degree. C. for the long-storage function; a temperature within the range from 0.degree. C. to 5.degree. C. should be maintained in the compartment intended for a short-term storage. A short-term storage is from 2 to 7 days, and the long-term storage up to ten months depending on the type of product.
Such temperature conditions are provided by using known cooling techniques.
One of the most energy-effective methods consists in obtaining cold using a vapour compression refrigerator unit in which a closed-circuit cycle of cooling agent circulation is employed. A cooling agent boils (evaporates) in an evaporator at a lower pressure P.sub.o and at a low temperature. The heat required for the boiling is taken off cooled objects so that their temperature decreases. The resultant vapour is taken off by a compressor, compressed therein to a condensation pressure P.sub.1 and fed to a condenser to be cooled with water or air. Owing to the heat removal from the vapour, the vapour is condensed. The resultant liquid cooling agent is returned back through a throttling member in which its temperature and pressure decrease to an evaporator for re-evaporation so that the cycle of operation of the refrigeration unit is thus closed.
It is generally known that, in order to improve the cost effectiveness of a compression refrigeration unit, its specific cold capacity should be increased, e.g. by improving the specific volumetric cold capacity of the refrigeration unit or by improving the volumetric efficiency of a compressor. It is also a common knowledge that the value of the volumetric efficiency of a compressor is inversely proportional to a suction pressure.
For achieving temperatures enabling freezing of products, that is -24.degree. C. and below, units with high P.sub.1 /P.sub.o ratios, hence low volumetric efficiency and low specific cold capacity are used in refrigeration apparatus.
Known in the art are methods of obtaining cold by using two- and multistage refrigeration units, comprising compressing a cooling agent from a boiling pressure to a condensation pressure not at once but gradually at two or several series connected stages with an intermediate cooling of partially compressed vapour.
The ratio of discharge pressure to suction pressure of a cooling agent at each stage is lower than the ratio of condensation pressure to boiling pressure, between which is effected the cycle of the staged unit (cf. French lay-open application No. 2182137, 1974, Cl. F 25 B 1/10, British lay-open application No. 1434927, 1976, Cl. F 4H).
A closed circuit cycle of a cooling agent circulation is employed in two- and multistage compression refrigeration units implementing the known methods of cold production, wherein a cooling agent comprises ammonia and fluorine-containing saturated hydrocarbons, mainly methane and ethane derivatives.
The prior art methods provide for producing cold at a temperature level of -24.degree. C. and below. Moreover, they also provide for obtaining temperatures within the range of 0.degree. to +5.degree. C. which are necessary for storing cooled products.
The prior art methods are, however, complicated because of a gradual compression of a cooling agent in several stages with an intermediate cooling of partially compressed vapour.
Sophisticated refrigeration units having several compressors, throttles, coolers, evaporators, and other devices are required for carrying out such prior art methods. Furthermore, the use of several compressors results in an increased energy consumption and lower reliability.
Also known in the art is a method for producing cold at a temperature level of 0.degree. C. to +5.degree. C. and -18.degree. C. and below using individual refrigeration units for each temperature range (cf. German lay-open application No. 2736370, 1979, Cl. F 25 B 49/00). Each refrigeration unit functions in accordance with a known vapour compression cycle using mainly difluorodichloromethane as a cooling agent.
However, this prior art method for producing cold exhibits a low specific cold capacity at the low-temperature level and requires much energy. This is due to the fact that, to obtain low temperatures (-18.degree. C.) and below, a greater difference in temperatures of boiling and condensation, hence a greater value of the ratio of the condensation pressure P.sub.1 to the boiling pressure P.sub.o, is required. With an increase in the ratio of these pressures, the volumetric efficiency and general efficiency of the compressor decrease to lower the specific cold capacity of the refrigeration unit and to raise the power requirements.
Also known in the art are methods of producing cold at a level of temperatures enabling cooling, freezing and storage of products with lower energy consumption, but such methods are associated with a complication of the cycle of a vapour compression refrigeration unit adding auxiliary operations with a cooling agent, and namely:
accumulation of a liquid cooling agent and its partial evaporation before the throttling stage (cf. U.S. Pat. No. 3,950,961, 1976, Cl. 62-149);
circulation of a liquid cooling agent along two circuits, each having an evaporator for evaporating the coolant agent at temperature levels of +5.degree., -0.degree. C. and -18.degree. C. and below (cf. British patent specification No. 1199267, 1970, Cl. F4H).
Though such methods can slightly reduce energy consumption, they require refrigeration units of a sophisticated design with lower reliability for their implementation.
At the same time, an optimum heat exchange process may be obtained using a cooling agent comprising a mixture of components to substantially improve the efficiency of compression and specific cold capacity of a refrigeration unit.
It is known that a high specific cold capacity may be obtained using mixtures of those cooling agents which have different boiling points. A particular feature of multicomponent cooling agents resides in that a high boiling component is condensed from a compressed mixture at the first condensation stage and a low boiling component is condensed at the second stage. The condensed components are caused to expand and boil at different temperature levels to provide required cooling and freezing conditions.
The employment of binary and multicomponent cooling agents makes it possible to obtain different boiling temperatures in evaporators without any auxiliary devices (cf. USSR Pat. No. 312429, 1971, Cl. F 25 B 5/00; USSR Inventor's Certificate No. 616493, 1978, Cl. F 25 B 1/00).
The prior art of this invention is a method for producing cold using a single-stage compression-type refrigeration unit functioning with a closed-circuit cycle, wherein a cooling agent in the form of a mixture of components boiling at different temperatures is compressed in a compressor to a pressure of 20 kg/cm.sup.2, partially liquefied by condensing a component with a higher boiling point, the mixture is completely compressed by cooling the forward flow by a reverse flow in a regeneration heat-exchanger, the components immiscible in the liquid state are mixed in a homogenization zone to obtain a homogeneous mixture, the resultant homogeneous mixture is throttled to a pressure of 3 kg/cm.sup.2, partially evaporated by evaporating the component boiling at a lower temperature in the evaporator zone, and the mixture is then completely evaporated by evaporating the component boiling at a higher temperature in the zone of the regeneration heat exchanger (cf. U.S. Pat. No. 3,872,682, 1975, Cl. 62-114).
The component boiling at a lower temperature is CO.sub.2 having a normal boiling point--79.8.degree. C., and the component boiling at a higher temperature is, e.g. difluorodichloromethane having a normal boiling point--29.8.degree. C.
This method makes it possible to obtain a temperature for cooling, freezing and storing products, but it exhibits a low specific cold capacity and considerable power requirements.
Known in the art are cooling agents for a vapour compression refrigeration unit functioning with a closed-circuit cycle, comprising a mixture of gaseous components, including ethane and propane (cf. USSR Inventor's Certificate No. 534484, 1976, Cl. C 09 K 5/00).
These cooling agents have an inadequate specific volumetric cold capacity and are less economical when a refrigeration unit functions in the range of compression pressures of 8 to 14 kg/cm.sup.2.
Also known in the art are cooling agents for a compression refrigeration unit functioning with a closed-circuit cycle, comprising difluorodichloromethane and a mixture of hydrocarbons: ethane in an amount of between 20 and 40 vol. %, propane in an amount between 10 and 30 vol. %, isobutane in an amount between 10 and 30 vol.%, and n-butane in an amount between 10 and 30 vol.%.
These cooling agents are, however, explosion- and fire-hazardous so that they cannot be used in home refrigerators upon which stringent requirements are imposed for explosion and fire safety.
Moreover, the employment of such cooling agents encounters serious difficulties in the batch production of refrigerators.